lighthouse: Gossip verify block to return block if no parent

Description

When verifying a block for gossip propagation, one error that can occur is ParentUnknown. In this case, we want to store the block and lookup its parents before processing a chain of blocks.

Currently this return type doesn’t return the block and we must clone the block before checking the verification in case we need to do a parent lookup.

To avoid the clone, the load_parent function should return the SignedBeaconBlock itself in the error message.